EIGHT FIGURESIn a recent paper (Allen et al., '53b) it was shown that exposure of newly metamorphosed toads to x-irradiation up to 20,000r did not decrease the capacity of their pituitary glands to secrete thyrotrophic hormone when they were transplanted into immature unirradiated tadpoles. On the other hand a dosage of 50,000 r of gamma irradiation very greatly reduced the secretory capacity. The thyrotrophic hormone secreted during the 20 days of the experiment was assayed by the growth of the hind limbs and the shedding of the cuticular teeth of the hosts as caused by the induced activity of the thyroid glands.I n the present paper the effects of dosages intermediate to those levels are covered. A more detailed study of the cytology of the transplants and of the stimulated thyroid glands of the hosts is also presented.It should be emphasized that this study does not concern the effects of irradiation upon the secreted thyrotrophic hormone, but upon the mechanism that secretes it since no irradiation is given after the transplantation of the gland.